In OneNote Online, there’s typically no need to manually save or sync your notes while you work. OneNote continually syncs your changes in the background, leaving you free to concentrate on your thoughts and ideas.
If you’re working on time-sensitive projects in shared notebooks that are simultaneously co-authored by multiple people, you may want to manually sync large page changes you make within any notebook section so that others can see them immediately. While this is not currently possible in OneNote Online, you can use the OneNote desktop app to manually sync notes when needed. For info about how to do this in OneNote 2016, see Sync a notebook.